Aspects of the present invention relate to Digital Subscriber Line (DSL) transmission. Other aspects of the present invention relate to multi-rate Symmetric DSL transmission.
Multi-rate symmetric DSL technologies usually use line probing at initialization time when a DSL connection is established. This implies that the negotiation and the selection of the transmission rate are based on the characteristics of the transmission environment detected during a relatively short period at the line probing during the initialization time.
The characteristics of a copper based transition system change dynamically over time and the changed characteristics affect the transmission environment. Examples of such characteristics include signal noise and cross talk. Some changes may lead to a worse transmission environment and some changes may lead to an improved transmission environment. For example, improved signal noise and/or reduced cross talk may improve the transmission environment. An improved transmission environment may represent an opportunity to upgrade a DSL connection to a higher transmission rate.
In current systems, symmetric DSL connections are not adaptive to the transmission environment. That is, the transmission rate after the line probing is fixed in a symmetric DSL connection. In asymmetric DSL connections, there are solutions that automatically adapt transmission rate according to the change in transmission environment. For example, signal to noise ratio is monitored after an asymmetric DSL connection is set up and running. Whenever the signal to noise ratio improves, the number of bits transmitted per symbol is automatically increased. Whenever signal to noise ratio degrades, the number of bits transmitted per symbol is automatically decreased.
Such adaptation in asymmetric DSL connections is usually performed without disrupting the connection when the transmission rate is being adapted. That is, the transmission rate is adjusted on the run without re-initializing the connection. One problem associated with this solution is that various parameters used in filters in both Customer Premise Equipment (CPE) and DSL Access Multiplexer (DSLAM) remain un-adjusted. This is due to the fact that no line probing or initialization is performed during the adaptation. This means that the automatic adaptation in current asymmetric DSL connections makes use of a gain obtainable in a filter setup that is adjusted according to the environment as it was by the time of the initial line probe. In this case, even though the transmission rate is adjusted to operate at an optimal speed, the transmission itself may not be operated with an optimal quality due to un-adjusted filter parameters.
Some other DSL arrangements allow a user to re-start the line probing process. With such a human-controlled re-start, operation of the connection is usually not based on the dynamic information about the current transmission environment.